Processes for the manufacture of fibers from molten mineral compositions, such as glass fibers, are well known and widely used. In these processes, mixtures of vitrifiable mineral compositions are typically heated in a furnace to a temperature sufficient to melt the mixture and lower the viscosity of the molten composition to a level suitable for fiber formation. Not unexpectedly, heating such compositions to such levels is an energy intensive operation that significantly effects the economics of the manufacturing process. Accordingly, anything that negatively impacts on the efficiency of the furnace in melting the mineral compositions is advantageously avoided.
The presence of impurities in the batches of vitrifiable materials fed to the furnace are known to negatively impact the melting efficiency of the furnace. In this regard, the sources of such impurities can take many forms, as can the mechanism by which they impact the furnace's melting efficiency. For example, the mineral compositions as received from the supplier themselves contain various impurities, in particular, sulfates, sulfides and organic contaminants. When conditions in the furnace are not suitably maintained, the dissolved sulfur species can evolve from the melt in a gaseous form causing a foam layer to form on the surface of the molten glass bath. This phenomenon causes the temperatures within the glass bath to increase due to the insulating character of the foam, and causes a reduction in the heat transfer rate from the molten glass bath to the unmelted batch which floats atop the foam layer. The results is a reduction in the melting rate of batch and an increase in the wear rate of refractories. The presence of organic impurities, such as wood, coal, oil residues and plastics further aggravates the deleterious effects of sulfurous impurities by reducing the activity of oxygen in the molten glass. The lower activity of oxygen causes dissolved sulfates to become less soluble, further increasing the formation rate of foam. For this reason, it is desirable to maintain a high oxygen activity in the melting system to maintain a high sulfate solubility in the glass and rid the batch of organic impurities through oxidation.
To create an oxidizing environment in the furnace, U.S. Pat. No. 5,346,864 suggests that a specific mixture of oxidizing agents be added to the mineral composition. In particular, it is suggested that both an inorganic nitrate, such as sodium nitrate, calcium nitrate or ammonium nitrate, and a second oxidizing agent selected from an oxide of manganese, potassium dichromate and/or ceric oxide be added to the mineral composition. However, while such nitrates generally dissociate at temperatures far below the melting point of the mineral components and therefore generate an oxidizing environment capable of removing many organic impurities present in the glass batch before reaching the molten glass bath and thereby helps to avoid the formation of a vaporous insulating layer between the molten and unmelted mineral batch, the potential for environmentally unfriendly No.sub.x emissions can severely limit the use of such nitrates.
Accordingly, a need exists for an oxidizing agent that decomposes at a temperature sufficiently below the melting point of the mineral materials to facilitate the oxidation of organic impurities before the glass melts and prevents the liberated oxygen from reaching the organic impurities, and to allow the gaseous oxidation products generated thereby to escape through the unmelted batch and avoid the creation of an insulating foam layer between the molten glass and unmelted batch. Further, a need exists for an oxidizing agent that does not cause the generation of environmentally unfriendly No.sub.x emissions and which does not introduce any residues that negatively impact the properties of the resulting glass. Such needs are met by the invention described herein.